Your child's account is Momio Lite because he/she or you as a parent have not given Momio consent to save some of your child's personal information. That's why your child can only send emojis.
By giving consent, your child can use all of the features in Momio as usual. If your child is under the age of 13, the consent must be given by a parent. If he/she is 13 or older, your child can give consent for their own Momio account.
In either case, consent is given in the Momio settings, which you will find on your child's profile by clicking the wheel icon and selecting "Account".